I had backups of my virtual machines in *.vma format (uncompressed). If your backup is compressed, you first need to uncompress it to get the vma-file.
vma is a special proxmox backup file which has to be extracted too. The easiest way for me was this python project: vma-extractor on Github. I used my working python install in Linux, you’ll fin executables for other systems here: python.org